This invention relates generally to a pixel circuit, and, more particularly, to a low-power organic light emitting diode (OLED) pixel circuit utilizing complementary metal oxide semiconductor (CMOS) technology.
An OLED pixel may utilize any of a variety of organic materials that emit light when an electric current is applied thereto. An OLED display comprises a plurality of OLED pixels organized into an array.
Illumination of an OLED pixel is controlled by a pixel circuit that may include either a constant current source or a constant voltage source. It is generally recognized that the constant current source provides a greater uniformity of luminance among the pixels of the array. This is because the dependence of luminance upon current tends to be uniform while the voltage across the OLEDs at a given current in the various pixels tends to be less uniform. U.S. Pat. No. 6,023,259 to Howard et al. describes a current driver that provides a passive matrix drive current to an OLED. For typical display brightness, passive matrix drive operation results in lower OLED power efficiency and, in order to avoid flicker, requires a refresh rate greater than or equal to 60 Hz.
In an active matrix display, a provision is generally made for storage of the state of a pixel within its respective pixel circuit. This is commonly achieved by incorporating the equivalent of a dynamic RAM cell in each pixel circuit, in which the state is stored as a voltage across a capacitor. A disadvantage of such an arrangement is that the voltage quickly bleeds off the capacitors, and consequently any image represented on the display must be regularly refreshed. This act of refreshing the image consumes a significant quantity of power. It is therefore desirable to find an alternative to the conventional use of dynamic RAMs.
Control of the luminance of an xe2x80x9conxe2x80x9d pixel is commonly achieved by controlling a magnitude of an analog voltage that determines the voltage or current applied to the pixel. Analog control circuits are another source of excessive power dissipation. It is therefore desirable to find an alternative to the conventional use of analog control circuitry.
The OLED display is constructed of thin layers of organic material in which individual OLED pixels are subject to an occasional short circuit between an anode of the OLED and a counter-electrode of the OLED. Such a short can cause excess current to flow in the circuit, excess voltage across the pixel circuit and excess power dissipation. Good pixels neighboring a shorted pixel may be destroyed by heat from the shorted pixel and the excess current may alter power supply voltages.
During operation of the OLED display, the layers of organic material tend to trap charge, causing increases in the voltage drop across the OLED, which can cause a non-uniformity of luminance and a burned-in image. The trapped charge may be removed by reverse-biasing the OLED.
The OLED itself typically requires a voltage on the order of +/xe2x88x926 volts to develop an adequate luminance when turned ON and to remove trapped charge when reverse-biased. Conventional CMOS integrated circuit technology uses transistors that operate with a voltage of less than 4 volts. Accordingly, CMOS technology is not ordinarily capable of driving an OLED. Furthermore, a CMOS device in an OLED pixel circuit is particularly susceptible to damage from the voltage in excess of 4 volts.
A traditional manner of changing a displayed image is for a processor to update the memory of a display controller that periodically and individually addresses each of the pixels of the display, and turn them xe2x80x9conxe2x80x9d (ON) and xe2x80x9coffxe2x80x9d (OFF) as required. If the display included a large number of pixels, for example one hundred thousand, one million, or more, this operation would consume a significant quantity of power and could burden the processor.
An additional problem when incorporating a plurality of pixel circuits into a display is that of physically distributing the collective elements of the display. That is, the display is a finite area within which the pixels and their accompanying circuitry are confined, yet a constant pitch between pixels must be maintained in order to provide a uniform image.
Because of the aforementioned disadvantages, OLED displays have not been as readily adopted by designers as have many other conventional display technologies. The use of dynamic RAM and its corresponding circuitry required for refreshing an image, and issues relating to power dissipation are an obstacle to an employment of OLEDs in battery operated devices, and in small displays such as those used for hand-held devices or watches. The OLED operating voltages are an obstacle to the use of CMOS circuitry in an OLED pixel circuit. The regular addressing of every pixel in an array is an obstacle to the employment of OLEDs in large displays. The maintenance of a constant pitch between pixels is an important consideration in any display.
It is an object of this invention to provide an improved OLED display that overcomes the foregoing and other problems.
It is a further object of the present invention to provide an improved OLED pixel circuit that dissipates low power, and is therefore suitable for use in a small battery powered device.
It is another object of the present invention to provide such an OLED pixel circuit in which a state of the pixel is retained in a static storage cell or memory while maintaining high circuit density and low power.
It is another object of the present invention to provide an OLED pixel circuit that utilizes CMOS technology for controlling an OLED.
It is another object of the present invention to provide an OLED pixel circuit that confines an effect of a short-circuited pixel so that non-shorted pixels continue operating in a normal manner.
It is yet another object of the present invention to provide an OLED pixel circuit that is capable of handling a large voltage variation across the OLED between normal and reverse-biased states without subjecting the OLED pixel circuit transistors to an excess voltage.
It is a further object of the present invention to provide an OLED pixel circuit that is suitable for use in a large display format.
In accordance with a first embodiment of this invention, a pixel circuit includes an organic light emitting diode (OLED), and a static memory for storing data that represents an operational state of the OLED.
In accordance with a second embodiment of this invention, a pixel circuit includes an OLED, a complementary metal oxide semiconductor (CMOS) circuit for controlling the OLED, and a protection circuit for protecting the CMOS circuit from over-voltage conditions.
In accordance with a third embodiment of this invention, a pixel circuit includes an OLED, and a CMOS circuit for controlling the OLED. The CMOS circuit contains a current source constructed using a field effect transistor (FET) having a static gate to source voltage that is greater than a threshold voltage of the FET.
In accordance with a fourth embodiment of this invention a display includes an array of pixel circuits. Each of the pixel circuits contains an OLED, and a static memory for storing data that represents an operational state of the OLED.